1. Field of the Invention
The present invention relates to a radiological image diagnostic system that radiates a radiation such as X-rays on an examined body and visualizes the inside of the examined body and a data processing method thereby.
2. Description of the Related Art
Heretofore, a radiological image diagnostic system has been used for a system for photographing a tomographic image of the inside of an examined body. Its representative example is an X-ray CT system (for example, refer to a document, “Method for Suppressing Streak Artifacts in CT Resulting from Excessive Noise” on pp. 272 to 276 of Medical Imaging Technology, 21 (4), 2003 written by I. Mori and M. Kazama). In X-ray CT, X-rays are radiated on an examined body from an X-ray tube and the X-rays transmitted in the examined body are detected in an X-ray detector and a data acquisition system (DAS). Further, processing including a logarithmic transformation process is applied to an X-ray detected value by a detection system including the X-ray detector and DAS and projection data is acquired. The projection data is reconfigured and an X-ray CT image is generated.
However, in an ultralow dosage area of an X-ray detected value, there is a case that the number of detected X-ray photons is substantially zero and so-called clipping may occur.
The X-ray detected value will be described below. A digital signal output from DAS is the sum of a signal s proportional to the number of X-ray incident photons on the detector, an offset o when the number of X-ray incident photons is zero and noise n. Precisely, further, the sensitivity constant and others of the detector and DAS are also related, however, as they are not related to the object of the invention, they are omitted. A preparation unit is provided with a function for acquiring a value acquired by subtracting the offset o, that is, s+n. This processing is called offset processing or offset correction. “s+n” is the X-ray detected value. In the case of data acquired by further applying slight processing to the X-ray detected value acquired as a result of offset correction, it is also called an X-ray detected value in the invention if no logarithmic transformation is applied to the data.
As the noise n is a positive or negative indeterminate value, the X-ray detected value x may have a negative value in case the signal output s is sufficiently small in comparison with the noise n. In case the X-ray detected value x is a negative value, logarithmic transformation cannot be executed and logarithmic transformation itself has no physical meaning. In such a case, generally, the X-ray detected value x is clipped to be 1 which is the minimum value for processing of the X-ray detected value x in a radiological image diagnostic system.
FIG. 9 is an explanatory drawing for explaining the concept of clipping executed in case an X-ray detected value is in an ultralow dosage area.
In a graph shown in FIG. 9, an abscissa shows a data value on the input side of logarithmic transformation and an ordinate shows a data value on the output side of logarithmic transformation. A solid curve in the graph shows a logarithmic function D1. Further, distributional data in a direction of the abscissa is distributional data on the input side D2 of logarithmic transformation and distributional data in a direction of the ordinate is a distribution data D3 showing the output of the logarithmic transformation of the distributional data D2. That is, the distributional data D2 is equivalent to an X-ray detected value and the distributional data D3 is equivalent to projection data. Precisely, though D3 is not projection data and projection data is acquired by applying some processes to D3, D3 is regarded as projection data in the invention because the processes are not important in the invention and a problem of clipping in D3 is reflected in projection data.
In the distributional data D2, as logarithmic transformation cannot be applied to a part having a negative value in a part D2a having a value smaller than 1 and a value equal to or smaller than 1 even if it is zero is equal to or smaller than a minimum unit which the system can deal, these values are all raised to be 1. This raise is called clipping. Therefore, the distributional data D3 after logarithmic transformation is remarkably distorted and a mean value D4 of the distributional data D3 is raised from the logarithm D5 of the mean value of the distributional data D3. As a result of the clipping, a mean value of projection data is shifted from a true mean value to a lower value.
Generally, as a value of projection data is acquired by multiplying a scaling constant by the value of projection data after logarithmic transformation is applied to an X-ray detected value and inverting its sign, a mean value of projection data is underestimated under a condition in which clipping occurs in comparison with a mean value of projection data in case no noise is included. A component the value of which is larger of projection data means that larger X-ray attenuation occurs and a component the value of which is smaller of projection data means that smaller X-ray attenuation occurs. In an image acquired by a reconfiguration process using such projection data, a CT value of a part in which large X-ray attenuation occurs is shifted and shaded.
That is, as described above, a conventional type radiological image diagnostic system has a problem that the shift and shading of a CT value occur in an X-ray CT image by clipping and the problem is currently sufficiently avoided.
Therefore, the development of technique for avoiding the occurrence of the shift and shading of a CT value of an X-ray CT image by clipping even if an X-ray detected value is in an ultralow dosage area is expected.